The present invention relates to a device for detecting decreased pressures of tires mounted on a vehicle and a method thereof. More specifically, the present invention relates to a device and a method for accurately detecting the decreased tire pressures by detecting that the kind of tires mounted on a vehicle differs.
When the pressures of tires mounted on a vehicle are decreased, a fuel mileage is deteriorated and there is danger of provoking burst at high speed running. When the tire pressures are reduced, decreased pressure is detected, utilizing the change of properties of tires such as the decrease of the dynamic load radii of tires and the change of resonance frequency.
For example, a conventional detection device for the decreased tire pressures uses a principle that since the outer diameter of a tire (the dynamic load radius of a tire) is reduced more than that of a tire having normal inner pressure when the pressure of a specified tire of a vehicle is reduced, rotational angular velocity is increased as compared with other normal tires. For example, a method for detecting the decrease of inner pressure from the relative difference of the rotational angular velocities of tires uses the value obtained from following formula as a judging value;DEL={(F1+F4)/2−(F2+F3)/2}/{(F1+F2+F3+F4)/4}×100(%)(Japanese Unexamined Patent Publication No. 305011/1988). Wherein F1 to F4 are the rotational angular velocities of a front left tire, a front right tire, a rear left tire and a rear right tire, respectively.
However, when the kind of tires mounted differs, the decreased tire pressures cannot be accurately detected.
Accordingly, a method for distinguishing the abrasion of a tire and the kind of tires has been proposed.
The invention described in Japanese Unexamined Patent Publication No. 188114/1997 discloses a method for distinguishing the kind of tires mounted on a vehicle, for example, whether the tire is a tire for summer or a tire for winter, by comparing the inclination of a straight line which is obtained by plotting a ratio of the rotational speed of a front wheel to that of a rear wheel against the speed of a vehicle, with the data of a tire whose kind is known by preliminary measurement.
The invention described in Japanese Unexamined Patent Publication No. 78442/1999 uses relation between a ratio of the rotational speed of a front wheel to that of a rear wheel and the acceleration of a vehicle in order to detect the abrasion state of tire.
The invention described in Japanese Unexamined Patent Publication No. 79812/2000 discloses a method for distinguishing whether the tire mounted on a driving axle is a tire for summer or a tire for winter by utilizing the reciprocal number of a turning radius using the judging value of tire deflation (DEL), but the method is limited to a vehicle mounted with LSD (Limited Slip Differential).
The invention described in Japanese Unexamined Patent Publication No. 19435/2002 and Japanese Unexamined Patent Publication No. 181669/2002 uses relation between a slip ratio (a value obtained by dividing the difference between the rotational speed of following wheels and the rotational speed of driving wheels by the rotational speed of driving wheels) and the acceleration of a vehicle, in order to distinguish the kind of tires.
Japanese Unexamined Patent Publication No. 211220/2002 discloses a method for distinguishing the kind of tires in accordance with the size of unevenness of the judging value of tire deflation (the average of the absolute value of the judging value of tire deflation).
The ratios of the rotational speeds of the front and rear wheels of respective left and right wheels are not proportional to the speed of a vehicle. For example, the ratio of the rotational speeds of the front and rear wheels is larger at a rising gradient road surface than that at a flat road even if its speed is slow. Further, acceleration is small at the rising gradient road surface but driving force is large; therefore the ratio of the rotational speeds of the front and rear wheels is larger than that at a flat road and the ratio of the rotational speeds of the front and rear wheels is not proportional to the speed of a vehicle. Consequently, it cannot be distinguished that the kind of left and right tires is different even though relation between the ratios of the rotational speeds of the front and rear wheels of respective left and right wheels and the speed or acceleration of a vehicle is examined.
When the kind of tires differs depending on positions of a vehicle, in particular, the kinds of tires respectively mounted on the left and right driving wheels differ, the tire deflation cannot be accurately judged by the threshold for judging tire deflation in the case of loading the same kind of tires. Further, when the kinds of tires respectively mounted on the left and right driving wheels differ, driving properties differ at left and right; therefore there are problems such as the distance of breakage becomes longer, because slip is generated and braking operation does not work well. Consequently, it is required that it is alarmed to a driver that the kinds of tires respectively mounted on the left and right driving wheels differ, and the vehicle control system (such as ABS and TRC) is actuated in accordance with the different kind of tires.